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Exploring the Human Body

How much do you know about the workings of your body? The human body is endlessly fascinating, and a bit mysterious. But if it's been a while since your high school biology class, don't worry. We'll explore the systems of the body and find out how they all work together to make us tick. You'll be learning about the circulatory system, immune system, endocrine system, urinary system, digestive system, nervous system, respiratory system, and lymphatic system.

The Circulatory System

At the center of the human body — and your circulatory system in particular — is the heart, the fist-sized pump that pushes blood to all of your other body systems. The function of the circulatory system is to deliver oxygen and nutrients to your body and remove carbon dioxide and waste products through a system of veins, arteries, and smaller blood vessels. There are 20 major arteries, including the carotid arteries (in the neck), the axillary artery (in the arm), femoral artery (in the thigh), and the coronary arteries that supply blood to the heart. Major veins include the external and internal jugular veins, which bring blood from the brain back to the heart, and the subclavian veins that run down each side of the body. Oxygenated blood is carried out to your body through blood vessels that decrease in size from arteries to arterioles to capillaries. Once the oxygen is used up, the blood returns to the heart through venules (small blood vessels) and veins. All together, these blood vessels extend for 60,000 miles inside your body.

Circulatory System Conditions

Diseases of the heart and circulatory system cause nearly one out of every three deaths in United States. The four most common problems that can occur in your circulatory system are heart disease (which includes coronary artery disease and heart attack), heart failure, high blood pressure, and stroke. Heart disease can result in a heart attack if your heart is not getting enough blood. Heart failure occurs when your heart becomes too weak to pump blood effectively. High blood pressure is too much pressure inside your arteries, which can result in kidney failure or other heart health issues. Stroke is brain damage that occurs if an artery in your brain gets blocked, causing decreased flow.

The Respiratory System

Your respiratory system, which includes the lungs (1), trachea (2), bronchi (3), and diaphragm (4), works closely with your heart and circulatory system. When you breathe in, the air passes through your nose and mouth and down into your lungs through your windpipe. Inside your lungs, the air eventually winds up inside 600 million tiny sacs called alveoli. It is in these air sacs that oxygen is transferred to the blood and carbon dioxide is removed from the blood. The oxygen-rich blood goes back to your heart and the carbon dioxide is exhaled out of your body.

Respiratory System Conditions

The most common respiratory system problem is the common cold. This infection can be caused by more than 200 different types of virus. Pneumonia is a viral or bacterial respiratory infection that gets into your lung tissue. Asthma is a narrowing of the lung passages that may be caused by allergic reactions to triggers such as dust, pollen, or pet dander. Smoking is one of the worst things you can do to your respiratory system. Smoking can cause bronchitis, which means there's too much mucus in your lungs, as well as asthma, COPD, and lung cancer.

The Urinary System

The main components of your urinary system are your two kidneys (1), which drain urine through two tubes called ureters (2) into your bladder (3). Urine is excreted from your body through another tube called the urethra (4). Your urinary system works closely with your circulatory system and your digestive system. After your body has taken all the energy it needs from the food that you eat and liquids you drink, one of the waste products left over is urea. Urea is carried by your blood to your kidneys where it is filtered out and excreted through your urine.

Urinary System Conditions

Common problems of the urinary system include infections, blockage, and loss of function. Infections can occur in the kidneys, the bladder, or the urethra. In men, a gland called the prostate, which is part of the male reproductive system, can become enlarged and block urine flow. Stones can form inside the kidney (as pictured here) or the bladder and cause blockage as well as infection. Renal failure occurs when the kidneys can no longer filter waste products out of your blood. The renal system can also be affected by kidney cancer or urinary tract cancers.

The Digestive System

Your digestive system is made up of a series of hollow organs that connect to form a digestive tract that goes from the mouth to the anus. Food digestion starts in your mouth and then goes through your esophagus (1), stomach (2), small intestine (3), large intestine (4), colon (5), and rectum (6). Two other organs, the liver (7) and the pancreas (8), contribute important digestive juices. The purpose of your digestive system is to break down the foods you eat into molecules tiny enough to be absorbed by your circulatory system. These molecules can then supply your body with nutrients and energy. The upper parts of the system concentrate on digestion and the lower parts on waste removal. Waste from undigestible parts of food moves through the large intestine into the rectum and is then eliminated as a bowel movement.

Digestive System Conditions

Common digestive system problems include indigestion, constipation, gas, bloating, and diarrhea. Almost everyone has experienced these conditions at some point, and they are not usually serious. Possibly more serious are infections of the digestive system caused by contaminated food that result in more serious types of diarrhea and vomiting. Appendicitis is a colon infection that is common in children and may require surgery. Helicobacter pylori is a bacterial infection that can cause a peptic ulcer. Less common inflammatory conditions of the digestive system include ulcerative colitis and Crohn's disease. Cancers of the digestive tract include esophageal cancer, colon cancer, stomach cancer, rectal cancer, and anal cancer.

The Nervous System

Just as your heart is the center of your circulatory system, your brain is the center of your nervous system. Think of your brain as a three-pound supercomputer that runs all the other systems in your body. The spinal cord is an 18-inch bundle of nerves that carries and returns messages to the computer. Your entire nervous system is built on billions of nerve cells called neurons that communicate with each other through chemicals called neurotransmitters. The nervous system is divided into two parts, the somatic nervous system (which connects the brain and spinal cord with muscles and skin) and the autonomic nervous system (which controls internal body processes like breathing and digestion).

Nervous System Conditions

Common nervous system problems include injuries and infections. The nervous system disorder called cerebral palsy can occur before or during birth if the brain does not get enough oxygen. (The brain of a child with cerebral palsy is shown at left.) Your nerve cells are very sensitive and can die quickly if they are not getting enough support from your circulatory system. Mental illness can be caused by chemical imbalances of neurotransmitters in the brain that result in disorders like schizophrenia and depression. Epilepsy is an abnormal brain activity that can cause seizures. Brain cells that start to grow uncontrollably can form into a brain tumor. Other nerve disorders include Bell's palsy, Huntington's disease, and Parkinson's disease.

The Immune System

Your immune system is a complicated network of specialized cells, organs, and tissues that communicate with each other and work together to recognize and attack foreign invaders. These invaders could be germs such as viruses, bacteria, or fungi. When an invader gets into your body, your immune system uses your lymphatic system and your circulatory system to bring specialized white blood cells to the site of the invasion (one white blood cell is shown at left amid red blood cells). Your immune system then produces antibodies or special chemicals that attach to the invader to inactivate or destroy it. Some parts of the immune system include the thymus, which produces T cells that protect the body from infection; the spleen, which filters the blood and clears it of bacteria and viruses; and bone marrow, which produces new red and white blood cells.

Immune System Conditions

Problems can occur when your immune system is underactive or overactive. Some children are born with underactive immune systems, which can lead to life threatening infections. An infection with HIV, the virus that causes AIDS, can also result in not having enough immunity. Examples of an overactive immune system include allergic reactions and autoimmune diseases. An autoimmune disease occurs when your immune system mistakes normal body tissues for foreign invaders. Some common autoimmune diseases include rheumatoid arthritis, Hashimoto's disease, diabetes (left, a diabetes patient checks her blood sugar), lupus, psoriasis, and multiple sclerosis.

The Endocrine System

Your endocrine system is made up of all the glands in your body that secrete hormones, the chemical messengers that influence all the other systems in your body. The endocrine glands include the hypothalamus (1), pituitary gland (2), thyroid gland (3), parathyroids (3), adrenals (4), pineal body (5), and the reproductive glands (6). These glands control functions such as growth, energy level, mood, and reproduction.

Endocrine System Conditions

Endocrine system problems usually occur when one of the glands produces too much or too little of its hormone. For example, if the pituitary gland produces too much growth hormone it can result in gigantism, but if it produces too little, dwarfism may be the result. An overproductive thyroid can lead to weight loss and nervousness, while an underproductive thyroid can cause weight gain and fatigue. Too much adrenal gland hormone can cause bruising and muscle weakness; too little can cause abdominal pain and fatigue.

The Lymphatic System

Your lymphatic system is a network of tiny vessels that carries an important body fluid called lymph. The lymph system includes the tonsils, adenoids, thymus, and spleen. Lymph is a clear watery fluid that contains proteins and other important substances. An important part of the lymphatic system is the spleen, an organ located under the left side of your ribcage. The spleen works with the lymphatic and circulatory systems to remove worn-out red blood cells. Your lymphatic system helps your circulatory system by collecting and returning fluid that leaks out of the bloodstream and by carrying away waste products. The lymphatic system also works with your immune system to help you fight off infections.

Lymphatic System Conditions

If your lymphatic system is not working well, fluid can collect outside your blood vessels in your tissues and cause swelling. This swelling is called lymphedema. Lymph nodes are clusters of lymphoid tissue located mainly in the neck, groin, and armpits. These nodes may swell up when your body is fighting off an infection, but sometimes the infection overwhelms the nodes and they become a focus of infection. Tonsillitis is an example of lymphoid tissue in the throat that can become infected. Other problems of the lymphatic system are lymphangitis, inflammation of the lymph vessels, or lymphoma, cancer that originates in the lymphatic tissues. There are two types of lymphoma, known as Hodgkin's lymphoma and non-Hodgkin lymphoma.

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